An optical module is an optical component for optically coupling a semiconductor light-emitting element or semiconductor light-receiving element to an optical fiber. It is being widely used in optical communications and many other fields. In a computer system used for data communications, for example, a semiconductor light-emitting element module and a semiconductor light-receiving element module are mounted as a pair on a board. Optical modules of this type usually comprises an optical semiconductor element (a semiconductor light-emitting element such as a laser diode, or a semiconductor light-receiving element such as a photodiode, for example), a holder for holding the optical semiconductor element, and a receptacle core fixed to the holder for holding a ferrule of an optical plug being connected in a fixedly fitted manner, the optical semiconductor element being optically connected with the optical fiber in the ferrule when the optical plug is connected to the optical module.
The ferrule of the optical plug is a member for holding an end of the optical fiber along the central axis thereof. It is made of stainless steel in some cases, but more commonly made of a ceramic material such as zirconia, due to durability and reliability. The receptacle core, on the other hand, is required to have even better wear resistance since it is a member into and from which the ferrule is frequently inserted or detached when connecting and disconnecting the optical plug, and because the optical module is a component which is mounted on a board. In the prior art, therefore, a construction has often been adopted in which a ceramic sleeve made of a ceramic material such as zirconia, is press-fitted or adhered to the inner circumferential surface of a tubular core housing.
When a ceramic sleeve made of zirconia, which has a relatively high hardness and good wear resistance, is used as the receptacle core of an optical module, these ceramic components have to be machined into precise dimensions and press-fitted to a core housing. Thus, the ceramic sleeve tends to be extremely expensive. A construction in which a split sleeve made of phosphor bronze is inserted into the core housing may be adopted to achieve low-cost manufacture. Metallic sleeves, however, generally have poor wear resistance, compared with ceramic sleeves, and lack reliability and durability because they are apt to wear compared with ceramic ferrules. Furthermore, repeated attaching and detaching may cause the problem of changes in the amount of light coupled between the optical semiconductor element and the optical fiber.
As a method for solving this problem, an optical connector receptacle has been proposed in which the contact surface (the inner circumferential surface of the bore) of the receptacle core with which the ferrule makes contact is covered with a material having such a low friction coefficient that the dynamic friction coefficient thereof with the ferrule becomes less than 0.7 (Japanese Patent Publication No. 3-107807). In the disclosure, materials selected from a group consisting of TiN, zirconia, alumina, polytetrafluoroethylene, fluorine resin and MOS.sub.2, or a composite material containing such materials are cited as examples of coating materials having low friction coefficient. This technique is based on the concept that the use of a receptacle core whose contact surface with the ferrule has a low friction coefficient makes the insertion of the ferrule easy, resulting in reduced wear. The materials cited therefore include those having considerably low hardness values.
If a material having a low friction coefficient and also a low hardness, such as fluorine resin or MoS.sub.2, is used, however, the inner circumferential surface of the bore may be damaged as a ferrule having a relatively high hardness is inserted. Although TiN has a relatively high hardness, it is inferior to TiC by way of comparison based on the requirement that the material of the contact surface with the ferrule should has a low friction coefficient and a high hardness. Note that a TiN sintered bulk material has a Vickers hardness of 1900-2800 Hv and a friction coefficient of 0.49.mu., while TiC has a Vickers hardness of 3000-4000 Hv and a friction coefficient of 0.25.mu..
As described above, the receptacle is hard to be removed because it is permanently fitted to equipment, whereas the ferrule can be easily replaced since it is detachably connected to an optical fiber. Needless to say, both of the receptacle and the ferrule are desired to be subjected to less wear, but in terms of maintenance as the entire system, the receptacle should preferably be harder than the ferrule. In this respect, it is not desirable to cover the bore inner circumferential surface of the receptacle with a film having a low hardness for the above-mentioned reason.
Requirements for the receptacle include high accuracy in the manufacture of the bore inner circumferential surface, in addition to high resistance to wear. When forming a film, it is important to choose a material and a film-forming method to ensure controllability of film thickness and a consistent film thickness over a wide range. This is because the changes in inside diameter caused by uneven and consistent film thickness may greatly affect optical transmission efficiency.